1. Field of the Invention
The present invention generally relates to a combined solar thermal-photovoltaic concentrator system, in particular to an interchangeable and fully adjustable concentrated solar thermal and photovoltaic system.
2. Description of the Related Art
Using point-focus or linear-focus reflectors with sun-tracking ability to concentrate solar energy and provide higher energy output has been widely applied in the solar photovoltaic (PV) industry for electricity production and in the solar thermal industry for thermal energy and electricity production. Some technologies also combine PV and thermal energy production to provide both heat and PV electricity simultaneously. Conventional combined concentrated solar technology often places both PV cells and thermal receivers on or near the focus point (or focal line) of the concentrating reflectors. A typical configuration of such combined solar receiver includes PV cells placed on the focal line of a parabolic solar concentrator and a thermal collecting unit placed directly on the non-solar collecting side of the PV cells to take away residual heat from the PV cells and recycle the thermal energy. In this type of configuration, the thermal units are mainly designed to take away extra heat from PV cells; therefore, such system is not suitable for high-temperature applications. This is due to the specific temperature requirement of PV cell operation. PV cells are most efficient at lower temperature. The efficiency of PV cells goes down significantly when the temperature of a PV cell reaches approximately 70° C. and above. Moreover, the life of PV cells is drastically shortened in long-term elevated temperature operation. On the other hand, solar thermal energy is most applicable and efficient at high temperature. For example, to achieve higher efficiency, solar thermal absorption chillers which utilize heat energy to produce chilled water typically require a working fluid temperature above 100° C. Direct thermal use for industrial applications typically requires a fluid temperature above 70° C., and 100° C. or higher for steam generation. For electric power generation, the working fluid temperature requirement is typically 300° C. to 450° C. to drive steam turbines and 600° C. and above to achieve high thermodynamic efficiency in current generation Stirling engines. For high-temperature thermal applications, available solar thermal units typically cannot be combined with PV cells to take advantage of the additional electricity production benefits. The conflicting temperature requirements of solar thermal energy and solar PV technology limit the development of integrated solar PV and thermal energy utilization.
In terms of geometry of PV cell placement, conventional PV receivers in a typical thermal-photovoltaic concentrator system are placed on the sun-collecting side of the primary concentrator (reflector). This configuration limits the geometry and the number of PV cells that can be placed on the sun-collecting side. Moreover, for typical low-cost single-crystalline and polycrystalline PV cells, high concentration ratios provide only marginal efficiency benefit because of the increase in PV cell temperature and minimal efficiency improvement at high concentration ratios. For typical PV cells, lower concentrating ratios (below 10× sun for single-crystalline and polycrystalline PV cells) provide a higher efficiency and energy output benefit. This limitation in concentration ratio conflicts with the requirement of a thermal concentrator which typically utilize 20× and above for linear concentrators and much higher concentration ratios to achieve higher temperatures in dish type concentrators.
Combined solar thermal and electrical energy is a very efficient, diversified and desirable form of solar energy utilization, especially in areas where usable land resources are limited. Compared to the use of solar energy to produce electrical energy, using solar energy to produce thermal energy is more efficient (typically 60-80% efficiency for thermal production vs. 15-30% for electricity production). Although the direct use of solar thermal energy is most efficient, electricity is often required by most end-users because of its diverse end-use. There are many limitations in the simultaneous production of thermal and electrical energy; these limitations arise from the technical availability and suitable capacity application for various commercially available thermal-to-electric power generators. For example, large solar thermal power plants (20 MW to several hundred MW) that utilize steam turbines connected to electric generators require a large land area to harvest solar energy and are usually located in remote locations. These large plants typically do not provide simultaneous heat energy to end-users as it is not economical to transport heated fluids over long distances. For small-capacity power generation, for example, Stirling engines combined with parabolic dish solar collectors, the cost can be prohibitive and these systems generally do not produce heat simultaneously. Other small-capacity systems using heat engines, such as organic Rankine cycle (ORC) heat engines combined with linear parabolic troughs, typically have low thermal-to-electric efficiency. When land size is limited for solar energy production, for example, in the vicinity of cities or on the rooftop of a commercial or residential site, a system that combines PV electric power generation with solar thermal energy offers a flexible capacity, diversified end-use, energy efficient and cost-effective option for combined thermal and electrical energy production. Such a system should allow the end user to adjust the relative electric or thermal energy output depending upon the particular end use. If the solar industry can provide combined electrical and thermal energy with low cost, high efficiency and high temperatures, with a capacity range in the kilo-watt to several megawatt levels or higher, combined thermal-electric solar energy systems have a very large potential market. This market includes distributed energy for industrial, residential and commercial applications, as well as large-scale combined energy plants. A dual-use concentrated thermal and PV electricity combined system fills this technical and capacity gap.